Known turning tools include ones which carry a diamond-shaped insert with multiple alternately usable cutting edges. Using the two acute corners of such a diamond-shaped insert, a negative side cutting edge angle and a positive end cutting edge angle can be formed. Thus, one such tool can perform cutting of radially outer surfaces or end faces of workpieces, as well as contour cutting, so that these tools are now being widely used.
But since only two of the four corners of such diamond-shaped inserts can be used as cutting edges, diamond-shaped inserts are economically disadvantageous over triangular or square inserts.
On the other hand, triangular inserts are advantageous in that all of its three corners are usable as cutting edges. But if trials are made to perform cutting of radially outer surfaces and end faces of workpieces, as well as contour cutting with a single such triangular insert, the triangular insert can be supported on the side of the seating face of the holder along only one of its three sides. Thus, compared to diamond-shaped inserts, of which two sides defining an acute corner are restrained, such conventional triangular inserts can be supported less stably on the holder.
The following patent documents propose solutions to these problems.    Patent document 1: JP patent publication 2001-509087    Patent document 2: JP patent publication 10-328909